Method and apparatus for expanding tubes

ABSTRACT

Method and apparatus for expanding tubing, particularly metallic tubing having a U-shaped bend therein, wherein a plug is inserted into one end of the tubing and forced therethrough by fluid pressure. In order to facilitate passage of the plug around the U-shaped bend, the plug is provided with generally rounded (e.g., spheroidal) ends and a recessed central portion having a generally concave cross section. In this manner, the plug, under the influence of fluid pressure, can pass through straight sections of the conduit and can easily pass around a U-shaped bend therein without becoming jammed in the tube and without materially distorting the circular cross-sectional shape of the tube.

United States Patent [1 1 Halstead METHOD AND APPARATUS FOR EXPANDINGTUBES [75] inventor: Fred E. Halstead, Waynne, Ark.

[73] Assignee: Halstead Industries, Inc., Felienople,

[22] Filed: Mar. 20, 1972 [2]] Appl. No.: 236,313

[56] References Cited UNITED STATES PATENTS 942,184 12/ 1909 Persons29/523 1,818,593 8/1931 Walworth 113/118 2,914,839 12/1959 Schwenger72/370 Aug. 28, 1973 Attorney-Thomas H. Murray et a1.

[57] ABSTRACT Method and apparatus for expanding tubing, particularlymetallic tubing having a U-shaped bend therein, wherein a plug isinserted into one end of the tubing and forced therethrough by fluidpressure. in order to facilitate passage of the plug around the U-shapedbend, the plug is provided with generally rounded (e.g., spheroidal)ends and a recessed central portion having a generally concave crosssection. 1n this manner, the plug, under the influence of fluidpressure, can pass through straight sections of the conduit and caneasily pass around a U-shaped bend therein without becoming jammed inthe tube and without materially distorting the circular cross-sectionalshape of the tube.

8 Claims, 8 Drawing Figures Patented Aug. 28; 1973 3,754,430

2 Sheets-Sheet 1 FIG. 24 /4 'lNTEG/PAL 24 BEA/D5 l2 l8 l4 I? F/G'. 2.

k1 l 1 i 12/ l -*1" Ii: ii TUBE METHOD AND APPARATUS FOR EXPANDING TUBESBACKGROUND OF THE INVENTION While not limited thereto, the presentinvention is particularly adapted for use in the manufacture of heattransfer devices, such as radiators, condensers and the like wherein acoil of tubing carrying a heat transfer medium passes through openingsin a plurality of fins. In an extended surface coil of this type, thefins must be brought into close contact with the coil for the purpose ofconducting heat from the fins to the tube wall, or vice versa.

Methods employed in the past to achieve this close fin-to-tube contactincluded solder or zinc dipping, winding or pressing fins tightly ontothe tubes, or expanding the tube from within and forcing it against acollar at the root of each fin. This latter method involves stressingthe material from which the tube is formed beyond its elastic limit suchthat a permanent deformation takes place.

In the manufacture of heat exchangers of this type wherein the tube isexpanded, a plurality of U-shaped hairpin tubes are passed throughopenings in the fins such that adjacent, parallel leg portions areconnected at one end through an integral U-shaped hairpin bend while theother ends of the leg portions are open. Each hairpin tube is thenexpanded such that its diameter increases and its outer periphery isforced into snugabutting relationship with the openings in the finswhile the 180 bends in the U-shaped hairpin tubes remain substantiallyunexpanded. Following the expanding operation, U-shaped couplings orreturn bends are brazed to the open ends of the leg portions in order toprovide a continuous back and forth path for the flow of a heat transfermedium through the coil.

The expanding operation can be achieved by simply pressurizing eachU-shaped tube with a hydraulic fluid. The difficulty with this'method,however, is that the tubing becomes work-hardened when it is bent.Consequently, the area of the tubing adjacent the bend is harder thanthe remaining tubing and does not expand as readily. As a result, thetubing may not expand into close abutting contact with the fins at theend of the leg portions adjacent the bend. In order to overcome thisproblem, attempts have been made to force spheres or balls through thetubing under hydraulic pressure. It has been found, however, that asphere will become lodged in and will not pass around the U-shapedhairpin bend at one end of the tubing.

SUMMARY OF THE INVENTION- In accordance with the present invention,expansion of tubing used in heat exchangers is accomplished by means ofa specially-shaped, generally cylindrical metallic plug which is driventhrough the tube by application of hydraulic pressure. In order tosuccessfully force the plug around the U-shaped hairpin bend in thetubing, it has been found that it must be provided with generallyspheroidal or rounded ends and a recessed central portion having agenerally concave cross section. Furthermore, the radius of the concavemedian portion must be at least equal to or greater than the innerradius of the hairpin bend. It has been found that whereas a spherecannot be forced around a U-shaped hairpin bend in tubing under theinfluence of hydraulic pressure, a plug of the type described herein caneffectively and uniformly expand the straight-line sections of the tubebut not the U-shaped hairpin bend connecting them, thus avoidingunnecessary friction and possible binding while passing through the bentsection. The plug, in passing around the U-shaped hairpin bend, distortsit slightly; however this is unimportant since no fins are attached tothe tubing at this point.

The above and other objects and features of the invention will becomeapparent from the following detailed description taken in connectionwith the accompanying drawings which form a part of this specification,and in which:

FIG. 1 is a schematic illustration of a heat exchanger which can befabricated in accordance with the principles of the invention;

FIG. 2 is an enlarged view of the tubing extending through the heatexchanger of FIG. 1, showing the manner in which it is expanded intocontact with flanges on fins;

FIG. 3 is an elevational cross-sectional view of a U- shaped hairpinbend in tubing, showing the plug of the invention passing therethrough;

FIGS. 4A and 48 comprise cross-sectional views showing the configurationof .the tube along line IV-IV before and after the plug of the inventionpasses therethrough;

FIGS. SA and 5B are cross-sectional views taken substantially along lineV-V of FIG. 3 showing the crosssectional configuration of the tube alongline V-V before and after the plug of the invention passes therethrough;and

FIG. 6 is a cross-sectional view of a fixture for inserting the plug ofthe invention into one end of the tubing and for it forcing therethroughunder hydraulic pressure.

With reference now to the drawings, and particularly to FIGS. 1 and 2,there is shown a finned heat exchanger 10, such as a condensing orevaporating coil used in air-conditioning equipment. It comprises aplurality of parallel fins 12 preferably formed from aluminum, copper orsome other metal of high heat conductivity characteristics. Provided inthe fins 12, as best shown in FIG. 2, are flanged openings 14, theflanges 15 being long enough to accurately space the fins one from theother. Received within the openings 14 are the opposite, parallel legportions 16 and 18 of a number of generally U-shaped hairpin tubes 20.In this respect, the leg portions 16 and 18 are connected at adjacentends by an integral hairpin bend 22; while the opposite ends 24 and 26are open. As shown, a number of hairpin tubes 20 are passed throughaligned openings in the fins 12 with the outer diameter of the legportions 16 and 18 being slightly less than the diameters of theopenings 14 in the fins 12.

In order to complete the assembly and provide good heat conductionbetween the tubing and the fins 12, it is then necessary to expand theindividual U-shaped hairpin tubes 20 so as to increase the diameters ofthe leg portions 16 and 18. After the expanding operation, U-shapedelbows or return bends 28 are brazed or otherwise fastened to the end ofthe lower leg 18 of each U-shaped hairpin tube 20 and the upper leg 16of the next succeeding tube in order to provide a continuous coilproviding a serpentine path for a heat exchange medium passing throughthe heat exchanger.

As was mentioned above, it has been common in the past to expand theU-shaped hairpin tubes 20 by simply pressurizing them. However, due tothe fact that the tubes are formed from a single length of tubing whichis bent as at 22, the bent portion 22 becomes workhardened; and thiswork-hardening effect will extend for a distance along the straight legportions 16 and 18. As a result, when a hairpin tube of this type ispressurized to expand it, the softer portions in the straight legsections 16 and 18 will expand more readily than the work-hardenedportions adjacent the bend 22; and if the pressure is increased to thepoint where the workhardened portion will expand into contact with theinner periphery of the openings 14 in the fins 12, the pressure mayburst or rupture the softer portions of the tube. Furthermore, expandingtubes in this manner by pressurizing them is a time-consuming andrelatively costly process.

It is also known that a straight length of tubing can be expanded byforcing a ball or projectile through it under the influence of fluidpressure. When an attempt is made to force a sphere through tubes suchas those shown in FIG. 1, however, it is found that the sphere becomeslodged within the U-shaped hairpin bend 22. Accordingly, the use of asphere for expanding hairpin tubes such as those shown in FIG. 1 is anunacceptable procedure.

In accordance with the present invention, U-shaped tubes such as thoseshown in FIG. 1 are expanded by means of the shaped plug 30 shown inFIG. 3. This plug is driven through the hairpin tube by means ofhydraulic pressure in a manner hereinafter described. The shape of theplug 30 is designed to facilitate propelling it through the tube with aminimum of friction while it is performing the expanding function. Thisshape is further designed to allow the plug to travel through 180 turnsin the tube with a minimum of friction and distortion. In FIG. 3, theplug 30 is shown after it has traveled halfway through a 180 hairpin orU-shaped turn. The ends 32 and 34 of the plug 30 are rounded as shown,with the rounded surfaces being defined by a radius R, having a centerpoint 36, above the center point of the radius of the 180 turn. Moreprecisely, the ends 32 and 34 comprise truncated spheroids. Intermediatethe ends 32 is a concave central portion 40 which, in cross section, isdefined by a radius R whose center point coincides with the center point38 of the [80 bend. However, it is only necessary that the radius R, beat least equal to or greater than that of the radius of the lower, innerperipheral surface of the bend. With this configuration, the plug 30will engage the inner periphery of the tube at narrow circular surfaces42 and 44 as it travels through the straight-line leg portions 16 and18. The surfaces 42 and 44 are formed at the intersections of concaveportion 40 with ends 32 and 34 and-comprise very narrow, essentiallyflat areas extending around the plug 30. As the plug passes around the180 bend 22, it will reach a point midway around the bend where thesurface defined by the concave portion 40 engages the inner periphery ofthe tube. In contrast to a sphere, for example, the plug shown in FIG. 3will pass around the 180 turn. The diameter of the circular surfaces 42and 44 is, of course, greater than the inner periphery of the tubebefore expanding; and the inner diameter of the tube will approach thediameter of the circular surfaces 42 and 44 after the plug passestherethrough.

FIG. 4A shows the cross section of the tube at the 180 hairpin turnbefore the plug 30 passes therethrough; whereas FIG. 4B shows thecross-sectional configuration after the plug is forced through the tube.Note that the cross section is somewhat distorted in FIG. 48 after theplug passes therethrough. There is no expansion of the tube in adirection perpendicular to its axis; however there is some deformationof the 180 turn in the plane of the turn. This deformation is caused bya slight eccentricity in the profile of the plug as shown in FIG. 3.Without this novel profiled, it is not possible to force the plugthrough a 180 turn in the tube. Since it is not intended to attach finsto the U- shaped ends of the coil, the design of the plug is such as toenable it to glide through the hairpin turn 22 without expanding thehairpin significantly. The straight sections of the hairpin tube,however, are expanded uniformly as shown in FIGS. 5A and 5B whichrepresent the cross section of the tubing before and after the plug 30passes therethrough.

If it is assumed that the plug 30 is traveling around the bend in thedirection of arrow 31 in FIG. 3, and that the plug is in the midwayposition shown in FIG. 3, the straight tube section to the right willhave expanded to an inner diameter D2 since the plug has already passedthrough it. The straight tube section to the left, however, still hasits original inner diameter D1. Diameter D2 is substantially equal tothe diameters of circular surfaces 42 and 44. Note that as the plugpasses around the bend, the diameters of surfaces 42 and 44 are notparallel to the radius of the bend, but at angles thereto, whichaccounts for the fact that the tube is not expanded in the turn in thesame manner as it is in the straight tube portions.

A fixture for accurately feeding the plug of FIG. 3 into the open end ofthe tube and for forcing it through the tube under hydraulic pressure isshown in FIG. 6. The end 26 of a U-shaped tube, for example, is expandedas at 50 and is engaged by serrated gripper jaws 52. The jaws 52, whichare circumferentially spaced around the tube end 26, are carried withina barrel 54 having a forward end provided with converging side walls 56.The side walls 56 engage corresponding sloped surfaces on the gripperjaws 52 such that when the jaws 52 are forced forwardly (i.e., to theright in FIG. 6), they converge and grip the end of the tube 26. Theplug 30 is carried between the jaws 52 and is adapted to engage a seat58 on the end of a piston or plunger 60.

Surrounding the piston 60 is a second cylindrical piston 62 having anenlarged forward end 64 provided with a seal 66. The enlarged end 64, inturn, is adapted to slide within cylinder 68 and is provided at itsforward end with an annular flange 70 which fits into cooperatingnotches'in the gripper jaws 52. With this arrangement, forward movementof the piston 62 will also force the jaws 52 forwardly, causing them toconverge and grip the enlarged portion 50 of the tube end 26. Hydraulicfluid, such as oil or water under pressure, is adapted to be connectedto the fixture of FIG. 6 through fitting 72 from whence it passes intothe interior of the hollow cylinder 62.

In the operation of the device of FIG. 6, the jaws 52 will normally beretracted, as will the cylinder 62. With the jaws thus retracted, a plug30 is inserted into the forward opening of the fixture until it restsagainst the seat 58. Thereafter, the forward end of the fixture ispositioned over the enlarged portion 50 of tube end 26. Following thisoperation, suitable valving, not shown,

forces fluid under pressure through the fitting 72 and into the interiorof piston 62. In so doing, the pressure acting on the enlarged diameterend 64 of piston 62 forces it forwardly, as well as the jaws 52 whichengage and grip the tube end 26. Immediately thereafter, fluid pressureacting on the piston 60 drives it forwardly, thereby forcing the plug 30into the interior of the tube end 26. A seal 73 is provided between anenlarged diameter portion of piston 60 and the hollow interior of piston62. It will be noted that the piston 60 is provided with a central bore74. Hence, fluid will pass through this bore 74 and force the plug 30through the U- shaped tube after the tube end is engaged and the pluginserted. The fluid used to force the plug 30 through the tube drainsafter the plug passes through the tube and is recycled. The force on theplug may be enough to cause it to travel some distance after it leavesthe other open end of the tube. A suitable screen or basket is providedto intercept all plugs so that they can be recycled and used repeatedly.

Although the invention has been shown in connection with a certainspecific embodiment, it will be readily apparent to those skilled in theart that various changes in form and arrangement of parts may be made tosuit requirements without departing from the spirit and scope of theinvention.

I claim as my invention:

1. In the method for expanding the diameter of a hairpin tube having agenerally U-shaped bend therein, the steps of inserting a generallycylindrical plug into one end of the tube with the ends of the plugbeing rounded and the central portion of the plug having an annularrecess of concave cross-sectional configuration, and forcing a fluidinto one end of said tube after said plug has been inserted therein tothereby force the plug through the hairpin tube including the U-shapedbend therein' 2. The method of claim 1 wherein said plug slides withinsaid tube on annular surfaces defined between said rounded ends and saidcentral portion of concave cross-sectional configuration.

3. The method of claim 1 wherein said central por tion of concavecross-sectional configuration is defined by a radius substantially equalto or greater than the inner radius of said U-shaped bend as expanded.

4. A generally cylindrical plug adapted to be forced under hydraulicpressure through hairpin tubing having a generally U-shaped bendtherein, the ends of the plug being rounded and the central portion ofthe plug having an annular recess of concave cross-sectionalconfiguration which permits said plug to pass around said U- shapedbend.

5. The plug of claim 4 wherein said plug is provided with circularsurfaces between its rounded ends and said central portion of concavecross-sectional configuration, said plug sliding on said circularsurfaces as it passes through the interior of the tubing.

6. The plug of claim 5 wherein said circular surfaces have diametersgreater than the diameter of the tube before expansion.

7. A fixture for inserting a plug into one end of tubing and for forcingit through the tubing under fluid pressure, comprising an outer tubularmember carrying within the interior thereof converging jaws adapted toengage and grip the end of said tubing, means for causing said jaws toconverge and grip the end of said tubing, piston means carried withinsaid converging jaws and adapted to force a plug also positioned betweenthe jaws into the end of said tubing, an opening extending through saidpiston means along the axis thereof, and means for forcing fluid underpressure through said opening for forcing said plug through the tubingafter it has been forced into said end of the tubing.

8. In the method for expanding a hairpin tube having a U-shaped bendtherein, which comprises forcing a plug through the tube with the plugbeing shaped such that the straight leg portions of the hairpin tube areexpanded in diameter, while the diameter of the U- shaped hairpin bendtherein remains substantially unchanged.

1. In the method for expanding the diameter of a hairpin tube having agenerally U-shaped bend therein, the steps of inserting a generallycylindrical plug into one end of the tube with the ends of the plugbeing rounded and the central portion of the plug having an annularrecess of concave cross-sectional configuration, and forcing a fluidinto one end of said tube after said plug has been inserted therein tothereby force the plug through the hairpin tube including the U-shapedbend therein.
 2. The method of claim 1 wherein said plug slides withinsaid tube on annular surfaces defined between said rounded ends and saidcentral portion of concave cross-sectional configuration.
 3. The methodof claim 1 wherein said central portion of concave cross-sectionalconfiguration is defined by a radius substantially equal to or greaterthan the inner radius of said U-shaped bend as expanded.
 4. A generallycylindrical plug adapted to be forced under hydraulic pressure throughhairpin tubing having a generally U-shaped bend therein, the ends of theplug being rounded and the central portion of the plug having an annularrecess of concave cross-sectional configuration which permits said plugto pass around said U-shaped bend.
 5. The plug of claim 4 wherein saidplug is provided with circular surfaces between its rounded ends andsaid central portion of concave cross-sectional configuration, said plugsliding on said circular surfaces as it passes through the interior ofthe tubing.
 6. The plug of claim 5 wherein said circular surfaces havediameters greater than the diameter of the tube before expansion.
 7. Afixture for inserting a plug into one end of tubing and for forcing itthrough the tubing under fluid pressure, comprising an outer tubularmember carrying within the interior thereof converging jaws adapted toengage and grip the end of said tubing, means for causing said jaws toconverge and grip the end of said tubing, piston means carried withinsaid converging jaws and adapted to force a plug also positioned betweenthe jaws into the end of said tubing, an opening extending through saidpiston means along the axis thereof, and means for forcing fluid underpressure through said opening for forcing said plug through the tubingafter it has been forced into said end of the tubing.
 8. In the methodfor expanding a hairpin tube having a U-shaped bend therein, whichcomprises forcing a plug through the tube with the plug being shapedsuch that the straight leg portions of the hairpin tube are expanded indiameter, while the diameter of the U-shaped hairpin bend thereinremains substantially unchanged.